Development of storage mediums on and from which information is recorded, reproduced and erased, particularly, magneto-optical disks characterized by large capacity and high-density recording has been carried out.
The magneto-optical disks are communicative. A conventional fixed hard disk has a closed structure. A fixed hard disk with a replaceable disk cartridge has been developed. In order to compete with such a fixed disk, a main object of development of magneto-optical disks is to realize a high-speed transfer device and high-speed access. In order to achieve the object, the development of a so-called overwrite technique is being actively carried out. The overwrite technique allows rewriting of recorded information by recording new information over the previously recorded information without erasing the previously recorded information.
A magnetic-field-modulation recording method is used for overwriting. With this method, recording and erasure of information on a magneto-optical disk are performed by reversing the direction of an external magnetic field using a magnetic head while applying laser light of a uniform intensity on the magneto-optical disk according to information to be recorded.
The magnetic-filed-modulation recording method uses a floating magnetic head or a fixed magnetic head. The floating magnetic head is mounted on a slider and pushed toward one of the surfaces of a magneto-optical disk by a suspension made of a plate spring. Therefore, when a flow of air occurs with a rotation of the magneto-optical disk, the magnetic head floats over the magneto-optical disk while keeping a uniform small gap between the magnetic head and the magneto-optical disk. Whereas, in contrast the fixed magnetic head is held in a position with a predetermined gap between the magnetic head and the magneto-optical disk so as to prevent the magnetic head and the magneto-optical disk from coming into contact with each other due to a vibration of a surface of the magneto-optical disk.
There is a magnetic field modulation recording method in which, when the rotation speed of a magneto-optical disk is not so high like a mini disk and a recordable and reproducible compact disk which is rotated at a speed of 190 to 400 rpm, the magnetic head is brought into contact with the magneto-optical disk, or the magnetic head is buried in a sliding member and the sliding member is brought into contact with the magneto-optical disk.
For example, as illustrated in FIG. 14, a conventional magnetic head 60 used in such a magneto-optical recording and is reproducing apparatus is constructed by winding a coil 62 around a magnetic core 61. For example, when the coil 62 is made of wire with 60 turns, the inductance is 30 .mu.H.
With the above-mentioned overwriting recording method on the magneto-optical disk, in order to modulate a magnetic field at a high speed, it is necessary to decrease the inductance of the magnetic head 60. Namely, it is necessary to reduce the cross section of the magnetic core 61.
Then, in order to modulate a magnetic field at a high speed without changing the magnetic field generating area, the development of a magnetic head having coils around a single magnetic core is being carried out.
However, the strength of an external magnetic field generated by the conventional magnetic head 60 is uniform and can not be varied. In the magnetic head having coils, in order to vary the external magnetic field strength using a conventional coil driving circuit, it is necessary to change the value of a power source voltage. In the conventional magneto-optical recording and reproducing apparatus, changing the value of the power source voltage is very difficult. Moreover, if the value of the power source voltage is to be changed, the size of the circuit needs to be enlarged, resulting in an increase in production costs.
In the conventional magneto-optical recording and reproducing apparatus, since the external magnetic field strength is not easily changed, it is impossible to apply magnetic fields of optimum strengths to disks having different magnetic field sensitivities.